The present invention relates to a signal transmission system having particular, but not exclusive application to the transmission of digitised analogue signals over a channel. The invention also relates to a transmitter and receiver for use in the system and to encoding and decoding arrangements embodied in that transmitter and receiver.
When analogue signals are to be transmitted or stored in a digital format they can be converted by an analysis means to a digital data stream comprising a number of digital values representative of parameters of the analogue signal. Examples of such analysis means are well known and include vocoders, the Group Speciale Mobile (GSM) speech coder and code excited linear prediction (CELP) systems. At least some of the parameters provided by these systems can be regarded as digital representations of time varying analogue signals, for example the spectral coefficients of a vocoder or the gain parameter of a CELP system.
For simplicity of expression, the word "parameters" and the phrase "digitized parameters" will each be used in the following description and in the claims to refer not only to the parameters of the analogue signal which are or have been measured or sampled, but also to the digital values of these parameters after measuring or sampling and conversion to digital form. Parameters of this type generally comprise a number of binary bits of which the most significant bit (MSB) carries the largest weighting and the least significant bit (LSB) carries the smallest weighting. Consequently when the analogue signals are resynthesised from their digital representations an error for example, in the MSB of a parameter has a much greater subjective effect upon the perceived signal quality than an error in the LSB of that parameter. For example the least important or significant bits may only slightly reduce the quality of the signals, for example speech, as reproduced when subject to an error rate of say 5% while the most sensitive or significant bits will introduce serious distortion when subject to an error rate of say 1%.
In circumstances where errors are likely to be introduced in the transmission or storage of digitised analogue signals, some means of error protection, such as redundancy, is added to the digital representations of the parameters. This protection usually enables the correction of up to a given number of transmission errors at the receiver. By protecting a number of parameters together, unrelated binary sequences are represented by a stream of output bits from a coder and this can mean that potential flexibility in the decoding process is lost. Additionally, simple block codes have traditionally been used for this purpose but these provide an equal amount of protection for all of the binary bits which they are used to protect and, in view of the widely differing subjective effects of errors in different bits of the parameters this approach is not regarded as being the most cost effective. United Kingdom Patent Application GB 2 182 529 A describes a method of encoding speech whereby block codes, which are designed to provide greater protection for the MSBs than the LSBs of the parameters, are used to encode one parameter at a time. This Patent Application also discloses a method of recreating the analogue signals from digital representations which contain errors by the use of a number of different techniques, including the use of known characteristics of each parameter, the previously received values of each parameter and soft decision information. Block codes which are suitable for use with this method are, however, difficult to derive and separate block encoding means must be provided for every different length of binary bit sequence to be protected.